Membranes have gained considerable importance as an inexpensive, low energy alternative to distillation for separation of gases. In particular, sieving of molecules based purely on size differences has emerged as a mechanism for obtaining extremely high selectivities of a particular component.
Currently, inorganic membranes constitute the bulk of separation materials, mostly for their stability at high temperatures. Other potential candidates for use as membrane materials include zeolites, polymers, ceramics and Carbogenic Molecular Sieve materials (hereinafter sometimes referred to as "CMS" or "CMS materials"). CMS materials have the advantage of being relatively inexpensive compared to zeolites, more temperature resistant than polymers and less brittle than ceramics Numerous studies have shown that a relatively narrow pore size distribution of 4-6.ANG. can be obtained by controlled pyrolysis of CMS precursor materials. Thus, it would be advantageous to utilize CMS in the form of a membrane to perform molecular sieving.
CMS materials can be derived from natural sources such as wood and coconut shells, as well as synthetic polymer precursors. The basis for their sieving action arises from the complex microstructure, which has been described as consisting of a network of aromatic domains and amorphous carbon. Disclinations between the various domains result in predominantly slit-shaped pores than can exclude certain molecules on the basis of size and shape. However, unlike zeolites, which have a unique pore size, CMS typically has a distribution of pore sizes that can range from 3 to 10.ANG.. one application of CMS is in the separation of nitrogen and oxygen using the pressure swing adsorption method. The kinetic diameters of the two molecules differ by a mere 0.2.ANG.--but careful control of the pore size results in very high selectivities for oxygen. This example also demonstrates the difference between a CMS, which performs true molecular sieving, and an activated carbon, whose performance is based on the difference in the adsorption equilibrium of gases. As nitrogen is more strongly adsorbed on activated carbon than oxygen, it would be held back and would have to be desorbed when the separation was complete. In a CMS, however, the equilibrium uptakes of both gases are the same--hence, the time of sieving becomes important to obtain a high selectivity.
CMS materials have been synthesized using a variety of different polymeric precursors. The controlled deposition of pyrolyzed carbon to narrow pores in activated carbons and other supports has also been studied extensively. Established synthesis methods involve pyrolyzing the precursor at a high temperature in an inert gas flow. However, not all polymers can be utilized for CMS production--this depends on whether they undergo cross-linking at high temperatures or not. The thermodynamically preferred structure for carbon at high temperatures is graphite. In the case of "graphitizing" polymers like PVC, graphite-like layers are formed at around 1000.degree. C., which results in a considerable decrease in microporosity of the material. Hence, the resulting carbon is not suitable for gas separations. On the other hand, PAN, PVDC and PFA cross-link at high temperatures to stabilize the structure and prevent the formation of graphite layers. This "non-graphitizing" character of the polymers is due to the presence of heteroatoms such as oxygen and nitrogen, as well as excess hydrogen. The pore sizes obtained are between 4-6 .ANG., which make them ideal for use as molecular sieves.
CMS materials are globally amorphous and do not exhibit any long range order as evident in zeolites. X-ray diffraction studies, which can resolve features on a length scale of 25 .ANG., do not reveal a distinct diffraction pattern for the microstructure. HRTEM studies of the structure combined with FFT analysis, can be used to determine the spacing between the graphite layers. The structure of CMS is thought to consist of a tangled network of ribbon-like aromatic regions. The evolution of the microstructure depends on the polymer precursor as well as the pyrolysis parameters of soak time and temperature. Investigations have shown that for most precursors, high temperature sintering leads to shrinkage of pores. There is, however, a collapse of the structure above a certain temperature, leading to a loss in the sieving property. A comprehensive review of CMS materials has been carried out by Foley (see Foley, H. C., Carbogenic Molecular Sieves: Synthesis, Properties and Applications; Microporous Materials, 1995;4; pp. 407-433).
There are two forms of CMS membranes--the unsupported "hollow fiber" form, and the supported form. The hollow fiber membrane was developed by Koresh and Soffer (see Koresh, J. E. and A. Soffer, Molecular Sieve Carbon Permselective Membrane Part I. Presentation of a New Device for Gas Mixture Separation; Separation Science and Technology, 1983; 18 (8); pp. 723-734) by pyrolysis of polyacrylonitrile (PAN) fibers. Despite their good sieving properties, the membranes lacked the requisite mechanical strength for use in various applications. A hollow fiber also cannot be converted easily into a module form that would be suitable for industry.
Supported CMS membranes can be synthesized using numerous techniques such as dip coating, spin coating, vapor deposition and sputtering. The ideal structure of such a membrane is shown in FIG. 4. It consists of a thin CMS layer 5 on top of a macroporous, non-selective support 7. The support provides mechanical strength to the membrane, which is a considerable improvement over the hollow fiber configuration. It also has the advantage of being available in various geometries such as flat plates, tubes and disks, which can be used depending on the requirements of the particular application. The support should be an inexpensive material and the pores in the support should be much larger than those in the CMS layer. For example, the pores in the support should be at least twice as large as the pores in the CMS material. In a preferred embodiment of the present invention, the pores in the support are from 5-500,000 times as large as the pores in the CMS material. In the most preferred embodiment of the present invention, the pores in the support are from 10 to 2,000 times as large as the pores in the CMS material.
Although the actual size of the pores in the various support materials can be widely varied, the nominal diameter of the pores in the support material should be greater than 100.ANG. (e.g., typical pore sizes in the support material are from 0.1 to 100 .mu.m in diameter). The size of the pores in the CMS material can also vary, but over a much narrower range. For example, the nominal diameter of the pores in the CMS material is generally from 3-100.ANG.. Preferably, the nominal diameter of the pores in the CMS material is from 3-20.ANG.. In the most preferred embodiment of the present invention, the nominal diameter of the pores in the CMS material is from 3-10.ANG.A.
CMS membranes have been successfully prepared on porous graphite and ceramic supports. These supports overcome the disadvantage of the hollow fiber configuration by providing durability to the membrane. However, neither of these materials is a good choice for process unit construction compared to metals and alloys. Further, the issue of forming a workable module of the composite membrane needs to be addressed. To successfully use the membrane, it must be put into a module that creates two zones for gas flow separated by the membrane. The critical parts of the module are the points of contact between the membrane and the module wall. These contact points are called end fittings or edge fittings in the case of a planar membrane. The fittings (seals) must provide complete isolation of the two sides of the membrane and should be devoid of any leaks that can create transport through a route other than the CMS layer. It is nearly impossible to fabricate leak free end fittings and modules for graphite and ceramic supported membranes. In the event that modules have been constructed, special end fittings were required, which would increase the cost if the process were commercialized. Thus, graphite and ceramic supports, while a definite improvement over hollow fiber membranes, are not able to meet the requirements of an industrial scale separation process.
One of the first attempts at making supported CMS membranes was by Bird and Trimm (see Bird, A. J. and D. L. Trimm, Carbon Molecular Sieves Used in Gas Separation Membranes; Carbon, 1983; 21; p.177). They pyrolyzed polyfurfuryl alcohol (PFA) on various support materials including silica frits, sintered bronze and copper and iron gauzes. Experiments were carried out to measure the diffusivities of various gases as a function of temperature. The researchers encountered the problem of being unable to create a uniform, defect free layer on any support surface, with the exception of silica frits. The control of the CMS microstructure was also very poor--membranes synthesized under similar conditions exhibited widely varying behavior in terms of gas diffusivities. However, there was some degree of separation obtained between gases, and this was attributed to flow through cracks as well as surface diffusion on the carbon. There was some evidence of activated diffusion as well, and activation energies were obtained for different gas-support material pairs.
Rao and Sircar (see Rao, M. B. and S. Sircar, Nanoporous Carbon Membranes for Separation of Gas Mixtures by Surface Selective Flow; Journal of Membrane Science, 1993; 85; pp. 253-264) developed the "Surface Selective Flow" (SSF.TM.) membrane, in which the primary mechanism for gas separation was the difference in surface flow of various gases on carbon. The membranes were synthesized by coating a layer of poly(vinylidene chloride)-acrylate terpolymer latex on a macroporous graphite disk with a pore size of 0.7 .mu.m. The samples were pyrolyzed at 1000.degree. C. in a nitrogen stream, and the coating procedure was repeated to increase the carbon layer thickness. SEM analysis revealed a crack-free membrane with a layer thickness of approximately 2.5 .mu.m. As compared to other separation mechanisms like Knudsen and molecular sieving, surface flow by selective adsorption was found to have several advantages. Components present in low concentrations could be separated, which eliminated the need for a large pressure drop across the membrane. Also, since surface adsorption increased at lower temperatures, ambient operating conditions improved the selectivity. The membrane was used to separate hydrocarbons from hydrogen and hydrocarbon mixtures and provided high selectivities for the former. Graphite supports were also used by Chen and Yang (see Chen, Y. D. and R. T. Yang, Preparation of Carbon Molecular Sieve Membrane and Diffusion of Binary Mixtures in the Membrane; Industrial and Engineering Chemistry Research, 1994; 33; pp.3146-3153) to synthesize membranes from polyfurfuryl alcohol (PFA). Again, the carbon layer was found to be crack free and its thickness was 15 .mu.m. Diffusivities of gases in the membrane were found to be concentration dependent. The experimental data was explained quite well by the binary diffusivity theory developed by the authors.